The Addon Manager doesn't have anything to do with this, because jetways are handled by FSX itself, it's basically the only thing in the scenery that the Addon Manager doesn't cover (which is why, it's the only thing left when the Trial time expires)
What you are seeing, is likely LOD optimization, jetways are made by different models in progressively less number of polygons, and the last LOD level doesn't have any polygons at all. It's an obvious frame optimization, it's made on purpose, and it's not possible to change it without recompiling the models and scenery.
We think the levels are appropriate but, you might see different results and earlier switches, if you fly with too wide zoom settings, since the LOD levels are affected by the zoom level. Flying at realistic zoom levels at around 0.70-0.75, should give normal results.
